1. Field of the Invention
This invention relates to a semiconductor memory system, and particularly to a memory system using a non-volatile semiconductor memory device for storing analog information.
2. Description of the Prior Art
The non-volatile semiconductor memory device in the prior art employs an MIS type memory transistor whose threshold voltage can be changed by injecting electric carriers into a gate insulator film or a floating gate or a bipolar memory transistor whose emitter junction is short-circuited by an input write signal applied externally. These memory transistors, however, store digital information only which consists of "1" or "0" value, and their use is limited to the digital information storage.
As memory devices in the prior art for storing analog information, there has been proposed a CCD (Charge Coupled Device) memory device which stores electric charges of different amounts in a semiconductor substrate under transfer gate electrodes. Since, however, the information storage in the CCD memory device is volatile, there is a limit in the storing time.
A use of non-volatile memory transistors in the analog information storage has been also proposed, in which an input analog information is once converted into digital information by an analog-to-digital (A/D) converter, and then digitally stored in the non-volatile memory transistors. The analog information is reproduced by reading the stored digital information and converting it into analog information with a digital-to-analog (D/A) converter. This memory system necessitates A/D and D/A converters and, therefore, is very complicated and expensive. Besides, the reproduced analog information through A/D conversion and D/A conversion is erroneous compared to the original input information.